1. Field of the Invention
This invention relates to a guide apparatus and a machine tool using the guide apparatus, which guides a movable member such as a column of the machine tool in a state of semi-contact.
2. Discussion of the Background
A machine tool has a bed, and a column that is moved on the bed, as main constituent elements. For instance, a workpiece is attached on the bed, and a machining means (a cutting tool) and a drive means (a tool drive motor) that drives the machining means are attached on the column. As a way to movably guide the column on the bed, it is known to supply pressurized lubricant oil between sliding surfaces of the bed and column, to keep a transition state which is intermediate between a state in which the column does not contact the bed (floats) and a state in which the column contacts the bed (does not float), so as to reduce weight of the column acting on the bed. In other words, a machine tool in which the column is moved under a state of semi-contact (semi-float) is known.
FIG. 1 is a side-view of a conventional machine tool. Referring to FIG. 1, a column 101 is moved (right and left directions in FIG. 1) along a sliding surface 111 of a bed 110. Lubricant oil is supplied between the sliding surface 111 of the bed 110 and a sliding surface 104 of the column 101. It is designed such that a product of an area of oil-hold portions (to be described later) which the pressure of the lubricant oil acts on, and the pressure of the lubricant oil, is equal to the weight of the column 101. Therefore, the column 101 can be guided on the bed 110 in a state of semi-contact (semi-float) by the lubricant oil. The column 101 is moved on the bed 110 in a state of semi-contact (semi-float) by a well known means such as a motor.
A tool spindle 105 supporting a tool, a tool drive means 106 to drive the tool spindle 105 and other elements are attached to the column 101. Accordingly, the center of gravity of the whole column 101 is unbalanced in a moving direction in most cases. Since such a load unbalanced column 101 inclines on the bed 110, the column 101 cannot be moved smoothly on the bed 110.
Therefore, a plural number of the oil-hold portions (for example, quadrangular grooves) to hold the lubricant oil are formed on the sliding surface 104 of the column 101, and an area of each oil-hold portion is designed according to a load which is acting on the oil-hold portion. For instance, two oil-hold portions 103a, 103b are formed on the sliding surface 104 of the column 101 along the moving direction of the column 104. The lubricant oil is supplied to the oil-hold portions 103a, 103b via throttle nozzles 102a, 102b. Areas of the oil-hold portions are designed according to the load of the column 101 acting at the location of each oil-hold portion 103a, 103b. That is, since the center of gravity 100 of the column 101 is located forward (left direction in FIG. 1) because of the locations of the tool spindle 105 and the tool drive means 106, the area of the oil-hold portion 103a onto which a heavier load acts is larger than the area of the oil-hold portion 103b onto which a lighter load acts. On the other hand, the throttle nozzles 102a, 102b are designed to have the same opening as each other and are connected to a single oil-supply device.
In the aforementioned machine tool, the same amount of lubricant oil, per unit time, is supplied to the oil-hold portions 103a, 103b whose areas are different from each other. When the stationary column 101 is moved, the column 101 is floated by an wedge effect of the lubricant oil, and a clearance between the sliding surface 104 of the column 101 and the sliding surface 111 of the bed 110 becomes wider. The wedge effect acting on the column 101 is remarkable in the state of the semi-contact (semi-float). The lubricant oil is supplied into the clearance from the throttle nozzles 102a, 102b. Since the openings of the throttle nozzles 102a, 102b are the same, supplying the lubricant oil to the oil-hold portion 103a whose area is larger than the area of the oil-hold portion 103b is delayed relative to supplying the lubricant oil to the oil-hold portion 103b. As the result, the pressure Pa of the lubricant oil in the oil-hold portion 103a becomes smaller than the pressure Pb of the lubricant oil in the oil-hold portion 103b. Therefore, the column 101 inclines further on the bed 110 and cannot be smoothly moved on the bed 110.
In view of the foregoing, it is an object of the present invention to provide an improved semi-contact guide apparatus and a machine tool using the guide apparatus. In order to achieve the above and other objects, the present invention provides a semi-contact guide apparatus comprising a first member, a second member, a plural number of oil-hold portions and an oil-supply device. The second member is movably guided on the first member. A sliding surface is formed on the second member. The plural number of oil-hold portions is formed on the sliding surface. Dimensions of the oil-hold portions are defined according to the load distribution of the weight of the second member. The oil-supply device supplies lubricant oil to the oil-hold portions. The amount of the lubricant oil supplied to the oil-hold portion is defined according to the respective dimensions of the oil-hold portions. Since the dimensions of the respective oil-hold portions are defined according to the load distribution of the weight of the second member, and the amount of the lubricant oil supplied to the oil-hold portions is defined according to the dimensions of the respective oil-hold portions, the second member is prevented from inclining on the first member, and the second member can be smoothly moved on the first member.